The present invention generally relates to an electric motor, and more particularly, to a linear motor most suitable for a positioning control, speed control or the like at high accuracy and also, to a linear driving system employing such linear motor.
Conventionally, a linear motor includes a field magnet in which N and S magnetic poles of permanent magnets are alternately arranged, and an armature coil. The motor is structured so as to relatively displace the armature coil or the field magnets by successively changing over the current to be supplied to the armature coil. The linear motor as described above may be classified into two types, the first a coreless type having only an air-core armature coil, and the second a cored type having windings wound onto projecting poles. In the above two types, the coreless type linear motor requires a large-sized stator yoke in order to obtain the field over an entire length of the linear motor, while the other cored type linear motor requires an armature core to form projecting poles for winding the armature coil. Both types having substantial problems for achieving compact size and weight reduction of the linear motors.
In order to overcome the disadvantages as described above, there has also conventionally been proposed a linear motor as shown in FIG. 8, which includes a movable member 6 having a magnetic pole iron plate 5, polyphase windings 4, and a stator field member 3 having field magnets 1 and a yoke 2, so as to freely move the movable member 6 with respect to the field magnets 1 through polyphase excitation of the polyphase windings 4. In the above known arrangement, as a method of reducing cogging produced in the movable member 6, alleviating the magnetic reluctance variation based on the displacement of the movable member, which a main cause for generation of cogging, is accomplished by devising shapes at the forward ends of the iron plate 5 in a similar manner as in the iron-core type linear motor, thereby to lower cogging generating level.
In the above known arrangement, cogging can not be fully reduced to a permissible level, since the magnetic pole iron plate 5 has the complicated shape at its forward end portions, and high accuracy is required for processing the shape. In other words, cogging becomes a serious cause of disturbance for effecting a constant speed control at high accuracy, while also presenting a large obstruction in the case where positioning control at high accuracy is to be effected.